1. Field of the Invention
The present invention relates to a nonvolatile memory circuit that can keep a stored logic state even after power is turned off. Further, the present invention relates to a memory device and a signal processing circuit including the memory circuit. Furthermore, the present invention relates to an electronic device including the signal processing circuit.
2. Description of the Related Art
A signal processing circuit such as a central processing unit (CPU) has a variety of configurations depending on its application but is generally provided with some kinds of memory circuits such as a register and a cache memory as well as a main memory for storing data or a program.
In a memory circuit such as a register or a cache memory, reading and writing of data needs to be performed at higher speed than in a main memory. Thus, in general, a flip-flop or the like is used as a register, and a static random access memory (SRAM) or the like is used as a cache memory. That is, a volatile memory circuit in which data is erased when supply of power supply voltage is stopped is used for such a register, a cache memory, or the like.
In order to reduce power consumption, a method for temporarily stopping supply of a power supply voltage to a signal processing circuit in a period during which data is not input and output has been suggested. In the method, a nonvolatile memory circuit is located in the periphery of a volatile memory circuit such as a register or a cache memory, so that the data is temporarily stored in the nonvolatile memory circuit. Thus, the register, the cache memory, or the like holds data even while supply of power supply voltage is stopped in the signal processing circuit (for example, see Patent Document 1).
In addition, in the case where supply of power supply voltage is stopped for a long time in a signal processing circuit, data in a volatile memory circuit is transferred to an external memory device such as a hard disk or a flash memory before the supply of the power supply voltage is stopped, so that the data can be prevented from being erased.